In recent years, in a cellular system for portable telephones and the like, a rapid increase in an amount of data traffic accompanying the wide use of smartphones has brought about a problem of a shortage of communication bandwidth in the cellular system.
To cope with such a situation, for example, it is considered in Long-Term Evolution-Advanced (LTE-Advanced), that is, the next mobile communication type, and later standards in the Third Generation Partnership Project (3GPP) that a small-scale base station apparatus (a pico base station apparatus that makes up a pico cell or a femto base station apparatus that makes up a femto cell) that has a smaller transmitted power and service area than a macro base station apparatus be additionally arranged within a service area of a large-scale base station apparatus (macro base station apparatus) that makes up a macro cell in the related art, for a local area where a large amount of traffic occurs in a concentrated manner. It is proposed that an increase in a capacity of communication of the system due to a traffic off-load effect be accomplished by such a heterogeneous network (HetNet) in which the small-scale base station apparatus is arranged within the service area of the macro base station apparatus (NPL 1).
In a general cellular system that is configured from the macro base station apparatus, in a case where the user equipment (the terminal apparatus) selects the base station apparatus as a connection destination, a received power of a reference signal (the received power of the reference signal is also referred to as a reference signal received power (RSRP)) that is transmitted by each base station apparatus serves as a reference and thus the base station apparatus that has a highest received power of the reference signal is selected as the connection destination.
However, for example, in the heterogeneous network in which the pico base station apparatus is additionally arranged within the macro cell, in a case where the base station apparatus that has a highest received power of the reference signal is selected as the connection destination, a difference in a transmitted power between the macro base station apparatus and the pico base station apparatus makes a cell radius of the pico base station apparatus considerably smaller than the macro base station apparatus and restricts an effect of increasing the amount of communication of the system due to the introduction of the pico base station apparatus.
Then, cell range expansion (CRE) is considered in which addition of an offset value to the received power of the reference signal from the pico base station apparatus increases a probability of the terminal apparatus selecting the pico base station apparatus as the connection destination and increases a cell radius of a coverage area (pico cell) of the pico base station apparatus (NPL 1).
For example, in LTE-Advanced, the offset value (CRE offset value) is notified to the terminal apparatus by high-layer signaling (NPL 2).
On the other hand, as a technology for greatly improving spectral efficiency, a multiple input multiple output (MIMO) technology that performs wireless transmission using multiple transmit and receive antennas is attracting attention, and is practically used in the cellular system, a wireless LAN system, or the like.
An amount of improvement in the spectral efficiency due to the MIMO technology is proportional to the number of the transmit and receive antenna. However, the number of the receive antennas that can be arranged in the terminal apparatus is limited. Thus, multi-user MIMO (MU-MIMO) in which multiple items of terminal apparatus that make connections at the same time are regarded as a virtual large-scale antenna array, and a transmit signal from the base station apparatus to each item of terminal apparatus is space-multiplexed is effective in improving the spectral efficiency.
In the MU-MIMO, because the transmit signals that are destined for the terminal apparatus, respectively, are received in the terminal apparatus, as inter-user-interference (hereinafter referred to as IUI), it is necessary to suppress the IUI. For example, in LTE or LTE-Advanced, linear precoding is employed in which the base station apparatus multiplies in advance a linear filter, which is calculated based on channel state information that is notified by each terminal apparatus, by the transmit signal, and thus the IUI is suppressed.
However, as long as channel orthogonality of each terminal apparatus, which is space-multiplexed, is not high, because the IUI cannot be effectively suppressed, there is a limit in the improvement in the spectral efficiency in the MU-MIMO that is based on the linear precoding.
Thus, in recent years, an MU-MIMO technique, which uses nonlinear precoding in which nonlinear processing is performed at the base station apparatus, has attracted attention. In the terminal apparatus, in a case where a modulo (surplus) operation is possible, it is possible to add to the transmit signal a perturbation vector of which a component is a complex number (perturbation member) that results from multiplying a constant real number by an arbitrary Gaussian integer. Then, according to a channel state between the base station apparatus and the multiple terminal apparatuses, the perturbation vector is suitably set and thus the transmit signal is generated. As a result, even though the channel orthogonality of each of the terminal apparatuses, which is space-multiplexed, is not high, it is possible to reduce necessary transmission power more greatly than in the linear precoding in which the perturbation vector is not added, and the transmission efficiency can be greatly improved (NPL 3 and NPL 4).